Vinyl acetate-ethylene (VAE) copolymers constitute a well known class of synthetic resins demonstrating a broad range of properties depending upon the relative quantities of copolymerized ethylene and vinyl acetate (and other ethylenic monomers which may be present) in the copolymer chain. Elastomeric amorphous VAE gum stocks contain from about 40% to about 70% vinyl acetate by weight randomly distributed throughout the copolymer chain and, when crosslinked, for example, by a peroxide crosslinking agent, possess properties which make them especially useful as elastomers for rubber compounding, as base copolymers for adhesive formulations and as impact modifiers for polyvinyl chloride (PVC). Among the physical and chemical properties which make the rubbery VAE copolymers attractive for such applications are the following: heat aging resistance; oil and solvent resistance; low compression set, good low temperature performance; excellent weatherability and ozone resistance; resistance to natural light; transparent or white-to-black vulcanizates; high loadability; receptance to dielectric heating; and high dampening characteristics. Thus, the elastomeric VAE copolymers are excellent candidates for such automotive applications as gaskets, seals and O-rings, wire insulation, radiator tubing and hose, bumper strips and auto body filler panels and are ideal for other demanding applications as well such as machinery mounts, weather stripping, washing machine hose, refrigerator gaskets, and the like.
In accordance with the present invention, VAE elastomers are obtained in the form of latices employing an improved emulsion copolymerization process and the elastomers are recovered therefrom employing such conventional techniques as coagulation. In general, a VAE copolymer latex is prepared by first charging an aqueous phase containing water, surfactant, buffer, catalyst or catalyst system of the free radical type, and usually a protective colloid such as polyvinyl alcohol (PVA), to a reactor as, for example, described in U.S. Pat. Nos. 3,708,388 and 3,714,096. In some procedures, an initial charge of vinyl acetate monomer, and in others, the entire amount of vinyl acetate monomer, is also charged to the reactor. The reactor is flushed with nitrogen, sealed, and stirring is commenced. Ethylene is then pumped to the reactor until the desired pressure is attained. The reactor can be repressurized one or more times if the batch is carried out under variable ethylene pressure, or a constant pressure can be maintained automatically employing techniques which are well known in the art. After reactor pressure has stabilized, the contents thereof are heated to the polymerization temperature, usually by circulating hot water or steam through a jacket surrounding the reactor. When the desired polymerization temperature (commonly from about 120.degree. to about 165.degree. F.) is reached, temperature is maintained at this level by automated controls. Thereafter, a co-catalyst such as sodium hydrogen sulfite (NaHSO.sub.3) can be added to the reactor (if a catalyst system employing a reducing agent to generate free radicals by a redox reaction is used) followed by any remaining vinyl acetate monomer. The completion of polymerization is indicated by cessation of ethylene demand and stabilization of the reactant coolant temperature at about 6.degree.-8.degree. F. above the reactor temperature. Upon completion of polymerization, the reactor contents are cooled and discharged through a pressure let-down valve to a receiving tank at atmospheric pressure from which unreacted ethylene is vented. The finished VAE copolymer latex is passed through a screen of desired mesh to complete the manufacturing process.
Various manipulations of both the amount and nature of the components of a VAE copolymerization medium and the copolymerization process variables have heretofore been attempted in order to optimize one or a few properties of the resulting latex. The emulsion polymerization process of U.S. Pat. No. 4,128,518 employs from 1 to 8% by weight based on the total weight of ethylene and vinyl ester monomers of a polyoxyethylenic nonionic surface active agent preferably one having an HLB (hydrophilic lipopylic balance) value of 5 to 20, as the dispersing agent for the reaction medium. U.S. Pat. No. 3,692,723 describes an emulsion polymerization medium containing a ternary emulsifier one component of which is a nonionic surface active agent having an HLB of from 16 to 21. The use of surface active agents having an HLB below 16 are to be avoided in the process described in U.S. Pat. No. 3,692,723 as an excessively large amount thereof would be required to provide sufficient emulsification effect whereas the use of surface active agents having an HLB higher than 21 are to be avoided in said process as they fail to provide a stable aqueous dispersion. U.S. Pat. No. 3,644,262 describes a copolymerization which by regulating the addition of vinyl acetate to an aqueous emulsifying composition containing a free-radical initiator at a rate which will maintain the concentration of unpolymerized vinyl acetate at a level not exceeding about 3.5% by weight of emulsifying composition and, optionally by delaying the addition of surface active agent, permits the introduction of substantially more ethylene into the copolymer for a given pressure and temperature than would be otherwise attainable. The resulting high ethylene content VAE copolymer latices are said to be better adapted to their end uses than the latices of relatively low ethylene content. A different approach to improved VAE copolymer latices is described in U.S. Pat. No. 3,423,352 in which high solids content VAE copolymer latices of reduced viscosity and improved freeze-thaw stability are obtained by controlling the addition of monomer, catalyst and surfactant. According to this patent, relatively large amounts of surfactant, i.e., from about 3% to about 10% by weight, and catalyst are added to a conventionally prepared polyvinyl acetate latex, having a solids content of up to about 52% and containing relatively large amounts of vinyl acetate, at specified times once polymerization has proceeded to a certain extent. This is said to result in a marked reduction in the viscosity of the emulsion. Frequently, these and other prior art techniques for preparing VAE copolymer latices achieve an improvement in one or two performance characteristics but at the expense of one or more other vital performance characteristics.
In accordance with the emulsion copolymerization process disclosed and claimed in commonly assigned copending U.S. patent application Ser. No. 005,070, filed Jan. 22, 1979, now abandoned, VAE copolymer latices are prepared by copolymerizing from about 60% to about 95% by weight of vinyl acetate with from about 40% to about 5% by weight of ethylene in an emulsion reaction medium containing a surface active agent in an amount of not less than about 1.0% by weight, and not more than about 2.0% by weight, of the total monomer, a catalyst and a protective colloid, the total weight of the surface active agent and vinyl acetate being introduced into the reaction medium in delayed increments prior to and after the commencement of copolymerization. The resulting VAE copolymer latices, which are employed as such as bases for paints and other surface coatings, as adhesives, textile treating agents, and the like, possess high inherent viscosity, i.e., not less than about 1.90, and demonstrate superior performance in the Time of Set Test and Vinyl Wetting Test.
Such properties are especially desirable attributes of a VAE polymer latex. However, where as here VAE elastomers are concerned, other physical properties, significantly, Mooney viscosity and gel content, are dominant considerations in the acceptability of the resins for rubbery articles such as enumerated above.